Mechanical gyroscopes are most commonly manufactured to include rotating structures. Spinning rotors have the advantage of producing large signals and, thus, provide high accuracy. However, the required bearings cause such instruments to wear out, and also make them unsuitable for the very low cost manufacturing techniques now available by micromachining silicon.
It has been recognized that vibration rather than rotation can also be used (Meredith, U.K. Patent 12539/42), and devices using vibration have been described (Morrow, Barnaby, and Nevala, U.S. Pat. No. 2,683,596). However, despite their long life, these instruments required such careful control of material uniformity and difficult mechanical balance that their cost-to-performance ratio was found to be uncompetitive with conventional rotating gyros.
There have been a number of attempts to construct micromachined gyroscopes using vibrating structures. Ljung (U.S. Pat. No. 4,884,446) and Greiff (U.S. Pat. No. 5,016,072) both describe structures which are potentially less expensive than those assembled from macroscopic parts. However, both of these approaches use angular vibration. The angular vibration is of restricted amplitude and consequently produces only very small Coriolis signals in response to rotation. As a result, both use resonant sensing elements at frequencies close to the vibration frequency in order to detect the small signals. However, this type of detection suffers practical problems. For example, the relative stability of the two frequencies affect scale factor, output noise results at the difference between the two frequencies, and a basic conflict exists between sensitivity and bandwidth. Also, both of these approaches use resonator structures which are difficult to directly fabricate to the high precision of balance needed for accurate gyroscopes. Any subsequent mechanical trimming is an expensive production process.
It is known that the vibration of a pair of accelerometers can form an instrument which will yield both gyroscopic and acceleration information. As well as the advantage of giving acceleration, which saves the expense of separate accelerometers in inertial applications, these instruments do not normally experience the difficulties associated with resonant sensing elements. Peters (U.S. Pat. No. 4,512,192) and Stewart (U.S. Pat. No. 4,744,248 and 4,841,773) reveal such structures that are assembled from macroscopic parts. However, their geometries are not suitable for fabrication by micromachining. Generically, such instruments giving both gyroscopic and acceleration information are known as multisensors.
It is known that an accelerometer structure can be fabricated from a body of semiconductor material, such as silicon, by so-called micromachining techniques. One suitable micromachining technique involves masking a body of silicon in a desired pattern, and then deep etching the silicon to remove portions thereof. The resulting three-dimensional silicon structure functions as a miniature mechanical device, for example an accelerometer that includes a proof mass suspended by a flexure.
In this regard reference is made to the following U.S. Pat. Nos. 5,006,487, "Method of Making an Electrostatic Silicon Accelerometer"; 4,945,765, "Silicon Micromachined Accelerometer"; and 4,699,006, "Vibratory Digital Integrating Accelerometer".
It is also known that gyroscopic information is available from vibrating accelerometers via the Coriolis effect. In this regard reference is made to the following U.S. Pat. Nos. 4,884,446, "Solid State Vibrating Gyro"; 4,841,773, "Miniature Inertial Measurement Unit"; and 4,744,248, "Vibrating Accelerometer-Multisensor".
U.S. Pat. No. 5,016,072, "Semiconductor Chip Gyroscopic Transducer", describes a micromechanical gyroscopic transducer that is formed from a mass of N-type silicon. Closed-loop rebalancing is said to be facilitated by the use of two quadrature servo-loops.
U.S. Pat. No. 4,512,192, "Two Axis Angular Rate and Specific Force Sensor Utilizing Vibrating Accelerometers", shows a two axis angular rate measuring system that includes two accelerometers having force sensing axes aligned at 90.degree. to each other and vibrating along an axis normal to the force sensing axes. A triaxial angular rate and force sensor is provided by combining two such sets of vibrating accelerometers, where the axes of vibration are normal to each other.